The Astrophysical Journal Letters, 799:L2 (7pp), 2015 January 20 doi:10.1088/2041-8205/799/1/L2C© 2015. The American Astronomical Society. All rights reserved.

OGLE-2013-SN-079: A LONELY SUPERNOVA CONSISTENT WITH A HELIUM SHELL DETONATION

C. Inserra1, S. A. Sim1, L. Wyrzykowski2,3, S. J. Smartt1, M. Fraser3, M. Nicholl1, K. J. Shen4, A. Jerkstrand1,A. Gal-Yam5, D. A. Howell6,7, K. Maguire8, P. Mazzali9,10,11, S. Valenti6,7, S. Taubenberger11, S. Benitez-Herrera11,

D. Bersier9, N. Blagorodnova3, H. Campbell3, T.-W. Chen1, N. Elias-Rosa10, W. Hillebrandt11,Z. Kostrzewa-Rutkowska2, S. Kozłlowski2, M. Kromer12, J. D. Lyman13, J. Polshaw1, F. K. Ropke14, A. J. Ruiter15,

K. Smith1, S. Spiro16, M. Sullivan17, O. Yaron5, D. Young1, and F. Yuan151 Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast,

Belfast BT7 1NN, UK; c.inserra@qub.ac.uk2 University of Warsaw, Astronomical Observatory, Al. Ujazdowskie 400-478 Warszawa, Poland3 Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK

4 Department of Astronomy and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720, USA5 Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel

6 Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102 Goleta, CA 93117, USA7 Department of Physics, University of California, Santa Barbara, Broida Hall, Mail Code 9530, Santa Barbara, CA 93106-9530, USA

8 European Southern Observatory for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2,85748 Garching b. Munchen, Germany

9 Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK10 INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy

11 Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany12 The Oskar Klein Centre, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden

13 Department of Physics, University of Warwick, Coventry CV4 7AL, UK14 Institut fur Theoretische Physik und Astrophysik, Universitat Wurzburg Emil-Fischer-Straße 31, D-97074 Wurzburg, Germany15 Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, The Australian National University Cotter Road,

Weston Creek, ACT 2611, Australia16 Department of Physics (Astrophysics), University of Oxford, DWB, Keble Road, Oxford OX1 3RH, UK

17 School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UKReceived 2014 October 22; accepted 2014 December 5; published 2015 January 9

ABSTRACT

We present observational data for a peculiar supernova discovered by the OGLE-IV survey and followed by thePublic ESO Spectroscopic Survey for Transient Objects. The inferred redshift of z = 0.07 implies an absolutemagnitude in the rest-frame I-band of MI ∼ −17.6 mag. This places it in the luminosity range between normalType Ia SNe and novae. Optical and near infrared spectroscopy reveal mostly Ti and Ca lines, and an unusuallyred color arising from strong depression of flux at rest wavelengths <5000 Å. To date, this is the only reportedSN showing Ti-dominated spectra. The data are broadly consistent with existing models for the pure detonationof a helium shell around a low-mass CO white dwarf and “double-detonation” models that include a secondarydetonation of a CO core following a primary detonation in an overlying helium shell.

Key words: supernovae: general – supernovae: individual (OGLE-2013-SN-079) – surveys – white dwarfs

1. INTRODUCTION

The observational and physical parameter space of knownsupernova (SN) types has recently been expanded by thediscovery of unusual optical transients. They are fainter andevolve on shorter timescales than normal Type Ia supernovae(SNe Ia) but are brighter than classical novae. Among them arebright and rapidly decaying objects like SN2002bj (Poznanskiet al. 2010), SN2010X (Kasliwal et al. 2010), and slowerevolving “Ca-rich gap” transients (Kasliwal et al. 2012) withMR � −16 mag, of which SN2005E represents the prototype(Perets et al. 2010). These objects are characterized by strongCa ii features in their spectra, but their physical origin remainsunexplained. It was initially suggested that some may beassociated with the detonation of a helium layer on a low-mass CO white dwarf (WD). Helium outer layers may buildup in binary systems in which a primary CO WD accretes froma degenerate (or semi-degenerate) He donor. If a sufficientlymassive He layer is accreted (�0.1 M�), detonation may occur.In the case of a low-mass CO WD, this may lead to a faintthermonuclear SN, roughly one-tenth the luminosity of a typicalSN Ia luminosity, hence dubbed “.Ia” (Bildsten et al. 2007;

Shen & Bildsten 2009; Shen & Moore 2014). A detonation ofthe He layer may also trigger a secondary detonation of theCO core, in a so-called “double detonation” scenario (e.g., Finket al. 2007, 2010; Kromer et al. 2010; Woosley & Kasen 2011;Sim et al. 2012; Shen & Bildsten 2014). Both scenarios showtitanium and calcium dominated spectra, peak magnitudes of−19 � MR � −17 together with rapid light curve declines(Shen & Bildsten 2009; Sim et al. 2012).

However, since none of these transients have unambiguouslymatched the theoretical expectations of pure helium shelldetonation or double detonation, alternative scenarios have alsobeen proposed. A large explosion of a massive star with severalM� of ejected mass may account for SN2010X and SN2002bj(Kleiser & Kasen 2014). The same interpretation is suggestedfor the rapidly evolving Type Ic SN2005ek (Drout et al. 2013).Similarly, the helium shell detonation scenario was suggestedfor SN2005E (Perets et al. 2010; Waldman et al. 2011), butalternatives have been proposed for SN2005E and Ca-rich gaptransients (see Kawabata et al. 2010; Valenti et al. 2014, andreference therein).

Here, we present the evolution of OGLE-2013-SN-079, a fastevolving SN with peak magnitude that lies in the luminosity

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Table 1Observed Photometry of OGLE13-079 and Assigned Errors

Date MJD Phasea g V r I z Telescopedd/mm/yy (days)

16/09/13 56551.15 −14.0 >22.48 OGLE20/09/13 56555.18 −10.0 21.28 (0.25) OGLE24/09/13 56559.15 −6.0 20.34 (0.07) OGLE27/09/13 56562.16 −3.0 20.10 (0.05) OGLE30/09/13 56565.19 0.0 19.87 (0.04) OGLE03/10/13 56569.32 4.1 21.16 (0.06) NTT05/10/13 56570.43 5.2 21.21 (0.16) 20.58 (0.03) 19.99 (0.04) FTS06/10/13 56571.16 6.0 20.05 (0.06) OGLE06/10/13 56571.42 6.2 21.41 (0.12) 20.72 (0.06) 20.09 (0.04) FTS07/10/13 56572.42 7.2 21.76 (0.08) 20.77 (0.02) 20.16 (0.04) FTS07/10/13 56573.03 7.8 21.40 (0.06) NTT08/10/13 56573.50 8.3 22.20 (0.18) 20.86 (0.07) 20.09 (0.10) FTS09/10/13 56574.44 9.2 22.41 (0.20) 20.89 (0.04) 20.18 (0.08) FTS09/10/13 56575.38 10.2 21.71 (0.05) 20.99 (0.06) 20.37 (0.06) NTT10/10/13 56575.50 10.3 22.69 (0.23) 21.03 (0.20) 20.54 (0.15) FTS11/10/13 56576.10 10.9 20.45 (0.08) OGLE16/10/13 56581.13 15.9 21.06 (0.20) OGLE20/10/13 56585.18 20.0 21.00 (0.24) OGLE24/10/13 56589.20 24.0 21.34 (0.26) OGLE24/10/13 56590.17 25.0 22.51 (0.16) 21.98 (0.14) 21.63 (0.16) 21.36 (0.16) NTT26/10/13 56591.07 25.9 >21.28 OGLE26/10/13 56592.19 27.0 22.62 (0.08) 22.09 (0.05) 21.86 (0.08) 21.67 (0.18) NTT30/10/13 56595.11 29.9 21.99 (0.34) OGLE07/11/13 56603.08 37.9 >22.00 OGLE11/11/13 56608.25 43.1 >23.40 23.20 (0.13) 22.65 (0.17) 22.35 (0.17) NTT

Deepest limits

25/11/13 56622.15 57.0 >23.45 NTT01/12/13 56628.11 62.9 >23.50 >24.00 >24.20 >23.50 NTT

Date MJD Phasea J H K Telescopedd/mm/yy (days)

13/10/13 56579.21 14.0 19.98 (.20) >19.60 >19.20 NTT25/10/13 56591.37 26.2 >20.10 NTT02/11/13 56599.31 34.1 >20.00 NTT

Note. a Phase with respect to the I-band maximum.

window −19 � MR � −15. It differs from previous transientsin showing titanium-dominated spectra and it appears to be avery promising helium detonation candidate.

2. OBSERVATIONS

OGLE-2013-SN-079 (hereafter OGLE13-079) was dis-covered by the OGLE-IV Transient Detection System(Wyrzykowski et al. 2014), at mI ∼ 21.3 mag, on 2013September 30.18 (magnitudes are reported in Table 1). Theobject coordinates have been measured on our astrometri-cally calibrated images: α = 00h35m10.s31 ± 0.s05, δ =−67o41′08.′′51 ± 0.′′05 (J2000). A spectrum, taken at the NewTechnology Telescope (NTT) + EFOSC2 on October 4.33 ut,as part of the Public ESO Spectroscopic Survey for TransientObjects (PESSTO),18 showed a red continuum with broad linessimilar to Type I SNe (Chen et al. 2013). The Galactic reddeningtoward the SN position is E(B − V ) ∼ 0.02 mag (Schlafly &Finkbeiner 2011). Since the available spectra do not show Na idlines related to internal reddening we will assume that the totalreddening is given by the Galactic contribution.

18 www.pessto.org

2.1. Host and SN location

On October 16.21 ut, we took PESSTO spectra of thetwo elliptical galaxies, probably interacting, at coordinatesα = 00h35m04.s47, δ = −67o41′02.′′7 (triangle in Figure 1)and α = 00h35m05.s21, δ = −67o41′14.′′7 (square in Figure 1),finding redshifts z = 0.074 and z = 0.072, respectively. Thesecond is also listed in the NASA/IPAC Extragalactic Databaseas 2MASXJ00350521-6741147 at redshift z = 0.071. If one ofthe two galaxies is the host, the transient would be at a projecteddistance of ∼49.9 kpc (∼35′′) from the first galaxy, or ∼40.2 kpc(∼30′′) from the second.

In our deepest images (VrI), we performed aperture photom-etry on the possible host (the closest). We found that the radiuscontaining half of the light of the galaxy is R1/2 ∼ 28 kpc, sim-ilar to the value R1/2 ∼ 30 kpc measured using the V magnitudeof the galaxy and a Sersic index of n = 4 (Graham 2013, andreference therein). Thus OGLE13-079 is likely located in theextreme outskirts of the host, in the halo region. Based on thisidentification, we adopt z = 0.07 as the redshift for OGLE13-079 throughout the following.

The host galaxy type and the remote location ofOGLE13-079 would argue against a massive star origin ofthis explosion (Hamuy et al. 1996; Anderson & James 2009).

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Figure 1. Left: FTS+Merope+g/r/i image of OGLE13-079 (cross marks). The sequence of stars in the field is indicated. The two closest galaxies to OGLE13-079are indicated (square and triangle). Right: NTT+EFOSC2+g/r/i image zoom of OGLE13-079 position (dashed cross). The reddish pattern is due to strong fringingof the i#705 filter.

Table 2Spectroscopic Observations

Date MJD Phasea Range Resolution Instrumentaldd/mm/yy (days) (Å) (Å) Configuration

03 Oct 13 56569.32 4.1 3700–9300 18 NTT+EFOSC2+gm1307 Oct 13 56573.05 7.9 3400–10300 11/16 NTT+EFOSC2+gm11/gm1615 Oct 13 56580.18 15.0 3400–23200 0.6/2 VLT+XSHOOTER+VIS/NIR30 Oct 13 56596.08 30.9 3400–23200 0.6/2 VLT+XSHOOTER+VIS/NIR

Note. a Phase with respect to the I-band maximum.

The offset distribution of 520 SNe published by Kasliwal et al.(2012) supports this idea. Massive stars could reach large dis-tances as a consequence of tidal stripping during galaxy inter-action but they should then be in the intracluster environment oftidal tails. We do not observe such tails in our data.

We note that the host and location of OGLE13-079 aredifferent from the bright and fast objects SNe 2002bj and 2010X,which both occurred close to the galaxy nuclei (Poznanski et al.2010; Kasliwal et al. 2010). On the other hand, Ca-rich objectslike SN2005E were found far from the nucleus (Kasliwal et al.2012; Yuan et al. 2013; Lyman et al. 2014). OGLE13-079 showsthe largest projected distance from its host nucleus among thesetypes of objects and it is comparable only to PTF09dav (Sullivanet al. 2011).

2.2. Data

Optical and near infrared (NIR) images were reduced(trimmed, bias subtracted, and flat-fielded) using the PESSTO(Smartt et al. 2014), the OGLE (Wyrzykowski et al. 2014), andFaulkes Telescope pipelines. Photometric zero-points and colorterms were computed using observations of standard fields (VIin Vega and grz in AB system). We then calibrated the mag-nitudes of a local stellar sequence shown in Figure 1. The av-erage magnitudes of the local-sequence stars were used to cal-ibrate the photometric zero-points in non-photometric nights.The JHK photometry was calibrated to the Two Micron All

Sky Survey system using the same local sequence stars. Ouroptical and NIR photometric measurements were performedusing the point-spread function (PSF) fitting technique. Differ-ences between passbands were taken into account (applying theS-correction; Stritzinger et al. 2002; Pignata et al. 2004).

All spectra (Table 2) were reduced and calibrated in thestandard fashion (including trimming, overscan, bias correction,and flat-fielding) using standard routines within iraf. The finalflux calibration was checked by comparing the integratedspectral flux, transmitted through standard Sloan or Bessellfilters, with our photometry. We applied a multiplicative factorwhen necessary, and the resulting flux calibration is accurate towithin 0.2 mag.

3. PHOTOMETRY

3.1. Light Curves

OGLE13-079 was observed during the rising phase only inthe I-band but subsequently followed in a range of opticalfilters until it disappeared beyond our detection threshold inlate November (Figure 2). The non-detection four days beforethe first observation places a strong constraint on the explosionepoch, allowing us to define the rise time and the initial shapeof the light curve. We estimate the explosion to have occurredat MJD 56553.5 ± 2, hence ∼11 days before I-band maximum.The post-peak decline has the same shape in each band with

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Figure 2. Top left: light curve evolution in optical bands compared with models. Open symbols denote limits. The vertical dashes indicate epochs with spectroscopiccoverage (black optical, red NIR). Top right: V − R evolution compared with models. Bottom left: OGLE13-079 r-band evolution compared with those of othertransients. Bottom right: bolometric light curve of OGLE13-079 compared with models. The shady gray area indicates the range of bolometric light curves underdifferent assumptions.

the exception of the g band, in which OGLE13-079 fades by1.5 mag over five days, in contrast to an average decrease of0.48 mag in the other bands. The slope in the VrIz bands isfairly constant until the last detection, with the exception of ashoulder in the I band at ∼15 days after peak that is reminiscentof the secondary maximum in type Ia (Kasen & Plewa 2007).

The OGLE13-079 magnitudes are comparable to SN2010Xbut ∼1.4 mag fainter than SN2002bj and ∼1.4 mag brighterthan SN2005E. Indeed, it is brighter than the most luminous(MR = −16.4 mag for PTF09dav, Kasliwal et al. 2012)of the Ca-rich transients. However, the post-peak decline ofOGLE13-079 of Δm15(r) ∼ 1.2 is similar to that of SN2005E(Δm15(R) ∼ 1.2) and slower than Δm15(R) ∼ 2.4 and ∼2.3of SN2002bj and SN2010X, respectively. Thus, while theOGLE13-079 r-band peak magnitude is similar to those of thefast evolving objects, the decline is comparable with the slowerones (Figure 2).

3.2. Bolometric Light Curve

To compare OGLE13-079 data with theoretical predictionsfrom explosion models (see Section 3.3), a bolometric lightcurve of OGLE13-079 was constructed using similar methods

to Inserra et al. (2013a, 2013b) and is shown in Figure 2. Weinitially built a gVrIz pseudo-bolometric light curve and addeda u contribution by assuming u − g ≈ 0, which is consistentwith the synthetic photometry retrieved from our early spec-tra. We added a NIR contribution assuming V − J ≈ 1.8 tobe constant throughout the evolution and adopting NIR colors(J − H and J − K) similar to the average of SNe Ia(Contreras et al. 2010; Friedman et al. 2014). Clearly, the lightcurve constructed via these steps has substantial uncertainties,therefore, we also construct a robust lower limit on the bolo-metric light curve by integrating only over the measured flux ingVrIz. We placed an upper limit on the bolometric light curve byusing the best-fit blackbody curve of the spectral energy distribu-tion retrieved through the red optical bands (rIz) by integratingfrom near ultraviolet to K-band.

The peak luminosity Lbol ≈ 1.4 × 1042 erg s−1 is roughly afactor ten and two less than in normal and faint (SN1991bg-like)SNe Ia, respectively.

3.3. Models and Theoretical Light Curves

We compared our photometric and spectroscopic data withthose predicted for the detonation of a 0.20 M� He layer around

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a 0.60 M� CO core presented by Shen et al. (2010, hereafter0.6+0.2 M�). We also compare to angle-averaged predictionsfrom 2D models computed by Sim et al. (2012): specifically,we chose one of their helium shell detonations (HeD-S, whichhas a 0.59 M� CO core plus 0.21 M� He layer) and one of theirdouble detonations (CSDD-L, which has a 0.59 M� CO coreplus 0.21 M� He layer). These were chosen because they givethe best match to OGLE13-079 absolute magnitude.

Of the models we considered, the best overall fit to the datais found with the CSDD-L model. However, the match is notperfect and, with the exception of the I band, the model is alwaystoo faint. We note that a good match of the I-band evolutionis also achieved by the HeD-S model but this model has abrighter and wider peak than the data or the CSDD-L model.The 0.6+0.2 M� model of Shen et al. is fainter and has a fasterdecline compared to the HeD model, ∼8-10d, depending on theband. This is likely a consequence of differences in the modelejecta structure and ionization state, which lead to differingdegrees of line blanketing in the blue and reprocessing of theUV/blue light by heavy elements. The 0.6+0.2 M� model doesproduce an I-band shoulder, although ∼10 days earlier thanobserved. We note that the rise time of OGLE13-079 is similarto those predicted by the models considered, while the “late”(>15d) decline has a comparable slope to the Sim et al. themodels. It is also noticeable that SNe 2002bj and 2010X aretoo rapidly fading compared to the models, while SN2005Eis too dim.

The best match of the bolometric light curve is also found withthe CSDD-L model, which is able to adequately fit the data from−6d to 16d from maximum. After that, OGLE13-079 declinesmore slowly for the next 10 days (15 � d � 25) but then settlesonto a decline slope similar to the model. The HeD-S model isbetween the limits of the bolometric light curves (gray area inFigure 2), but the fit generally appears poorer both around peakand at later times compared to the CSDD-L model. The Shenet al. model also fits the data reasonably well, although in thebolometric light curve of OGLE13-079 we do not observe anydouble-peaked behavior. Rise time and post maximum evolutiondifferences between data and models are roughly similar to thesingle bands previously shown.

3.4. Color Evolution Comparison

In the top right panel of Figure 2 we compare the V − R colorevolution of OGLE13-079 with those predicted by the models.Since we did not have R-band observations, we transformed rto R magnitude using snake (a python code for K-correctionand magnitude conversion; C. Inserra, in preparation). Althoughthe light curves are more similar to the CSDD-L model (seeabove), the V − R behavior is closer to the HeD-S model witha V − R ∼ 1.1 after 10 days since maximum. We note thatthe V − R evolution is also comparable to CSDD-L model butshifted by ∼0.3 to bluer values. Both the Sim et al. and Shenet al. models of pure helium detonation have similar behavior,although the Sim et al. models are systematically redder.19

4. SPECTROSCOPY

The rest-frame spectra are shown in Figure 3 and reported inTable 2. The striking features of the spectral sequence are thered color and the two prominent absorption profiles at 5300 Å

19 The redder colors of the Sim et al. compared to Shen et al. models are likelyattributable to a combination of differences in the treatment of ionization andejecta composition.

and 5900 Å. To our knowledge, such features have never beenobserved at an early phase in any other SN. By comparison withwavelengths and oscillator strengths from the Kurucz database(Kurucz & Bell 1995) we identified these as multiple, blendedTi lines. Similar lines have been predicted in helium shelldetonation models (Shen et al. 2010; Waldman et al. 2011). Thespectra are dominated by Ti ii lines during the 30d post-peakevolution; the majority of the line profiles bluer than 6000 Åare related to Ti ii lines,20 with the exception of the weak, orpossibly absent, Ca H&K lines. We note that sub-luminoustype Ia also show strong Ti absorptions in the blue. The Ca iiNIR triplet becomes more prominent from the second spectrumonward. However, it is weaker than the 5300 Å line and weakerthan typically observed in Ca-rich objects. In these objects theCa ii is roughly three times stronger than other features (seeValenti et al. 2014). The NIR absorption around 9800 Å couldbe attributed either to Ti ii line or Ca ii. We do not observe anyobvious lines associated with intermediate elements such as Si,S and light elements as Mg, C and O as seen in other possiblehelium shell detonation candidates. Ti ii and Ca ii are dominant,and consistent with what is expected in a helium shell detonation(see also Holcomb et al. 2013). We note that OGLE13-079 isthe first transient showing strong Ti lines, hence it could be thefirst example of what might be called a “Ti-strong” object.

4.1. Comparison with other HeliumShell Detonation Candidates

In Figure 3 we show OGLE13-079 with three other wellstudied transients of similar luminosity, SNe 2002bj, 2005E,and 2010X, at similar epochs. The spectra of SN2002bj arequite different from those of OGLE13-079: SN2002bj has amuch bluer continuum and does not show the strong Ti lines(around 5300 Å and 5900 Å). SN2005E has a similar red colorto OGLE13-079 but also does not show the two prominent Tilines. In contrast, SN2005E shows a strong Ca ii NIR tripletfeature, forbidden [Ca ii] 7291,7323 Å, and unambiguous He ilines. These led Perets et al. (2010) to identify the object asa Type Ib and a Ca-rich transient. We do not see forbidden[Ca ii] or any He i lines in OGLE13-079 spectra at comparablephases. OGLE13-079 has a similar color to SN2010X but thespectral features are again different. The spectra are particularlydistinct between 5500 Å and 7500 Å where Na i, O i and Mg i arenot present in OGLE13-079. SN2010X also shows a prominentCa ii NIR triplet, which is similar in strength to those of otherCa-rich events but is not visible in the OGLE13-079 spectrumwith comparable intensity. We note that the OGLE13-079 Ca iiNIR triplet velocity is ∼4000 km/s slower than those ofSNe 2005E and 2010X in the same phase. We conclude thatOGLE13-079 does not closely resemble any of these objects inits spectral features or colors.

4.2. Comparison with Double-detonationand Helium Shell Detonation Models

In Figure 3 we compare the OGLE13-079 spectrum withSim et al. and Shen et al. models at similar epochs (∼18 daysfrom explosion). The flux computed from the model spectrahas been scaled to match that of OGLE13-079. All lines andtheir absorption and emission strength are well reproducedby the Shen et al. 0.6+0.2 M� model. The model is slightlybluer, as also highlighted by the color evolution. The CSDD-L

20 We note that titanium production could be important for the positron budgetand 511 keV emission (Perets 2014).

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Figure 3. Top: optical (left) and NIR (right) spectral evolution of OGLE13-079. The ⊕ symbols mark the strongest telluric absorptions. X-SHOOTER spectra areconvolved with a Gaussian function of FWHM = 5 Å and subsequently binned to 5 Å per pixel. Middle: comparison between OGLE13-079 spectrum and models at∼8d after I-band maximum (∼18d from explosion). Bottom: comparison between OGLE13-079 spectrum and other transients.

model does a reasonably good job in reproducing the spectrum,although we note that it does not provide a good match tothe two noteworthy lines at 5300 Å and 5900 Å, which we haveattributed to Ti ii. These differences between the models could bedue to the distribution of Fe-group elements in the ejecta, whichare key to shaping the spectra (see Figure 6 of Sim et al. 2012),and generally to the treatment of ionization. The comparisonbetween the 0.6+0.2 and HeD-S models (similar masses butdifferent densities) shows that the Sim et al. models fit the color

better but the Shen et al. model fit the Ti ii lines quite well. Theexisting models do not predict strong He lines, however, wenote that further studies that include treatments of non-thermalexcitation are required to better investigate the formation ofHe lines in the optical (Hachinger et al. 2012). We note thatthe majority of model lines are either not observed or havedifferent strength (e.g., Ca ii) in the other transients mentionedin Section 4.1. Although OGLE13-079 light curves could, inprinciple, be fitted with a massive star explosion it would be

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difficult to reproduce an oxygen-free and titanium-dominatedspectral sequence.

5. CONCLUSIONS

OGLE13-079 is a transient in the luminosity region −19 �MR � −15 spectrophotometrically different from similartransients discovered so far. Its maximum luminosity MI ∼−17.6 is similar to SN2010X, thus brighter than the prototypeCa-rich transient SN2005E by ∼1.5 mag and less luminousthan SN2002bj by ∼1 mag. Its decline is comparable to thatof SN2005E and other Ca-rich transients. Its location in theoutskirts of the host galaxy (∼40 kpc), similar only to the Ca-rich object PTF09dav, makes OGLE13-079 the most remotegap object so far discovered. OGLE13-079’s spectral evolutionis unique since there is little or no trace of C, O, Mg, Si, Sin contrast to the other transients like SNe 2002bj, 2005E, and2010X. The OGLE13-079 spectra are dominated by He-burningproducts such as Ti ii and Ca ii. The titanium lines are noticeablystronger than any other elements and suggests that OGLE13-079could be the first “Ti-strong” transient.

OGLE13-079 is the first transient that reasonably wellmatches the synthetic observables predicted by models fordetonation of a He layer around a low-mass CO WD and/orequivalent double detonation models. The strong lines of Ti ii,a He-burning product, are reproduced by the nucleosyntheticreactions in these models. The lack of prominent features fromintermediate-mass elements in the spectra of OGLE13-079 isstriking. This combination is a quantitative signature of these ex-plosion models. Not all the observed characteristics of OGLE13-079 are well matched by these models. More focused and de-tailed theoretical simulations are warranted to investigate if thefull data set can be matched with a pure helium shell detonationor a double detonation of a lower-mass shell (<0.2 M�).

C.I. thanks M.Kasliwal (SN2010X data). Based on observa-tions at ESO as part of PESSTO (188.D-3003/191.D-0935/092.D-0555). Funded by FP7/2007-2013/ERC grant agree-ment [291222] (S.J.S.). We acknowledge support from STFCgrant ST/L000709/1 (S.J.S., S.S.), TRR33 grant of DFG(S.T.), FP7/2007-2013 grant [267251] (N.E.R.), FP7/ERCgrant [320360] (M.F.).

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